1. Field of the Invention
The present invention relates to thermosensitive recording materials having excellent thermal response and having minimized tailings or foreign matters adhered to the thermal head.
2. Discussion on Related Art
Thermosensitive recording materials are generally composed of a support having provided thereon a thermo-sensitive recording layer containing as major constituents an ordinarily colorless or slightly colored electron giving dye precursor and an electron receptive developer. When heated by means of a thermal head, thermal pen or laser beam, the dye precursor instantaneously reacts with the developer to form a recorded image, as disclosed in Japanese Patent KOKOKU (Post Exam. Publications) Nos. 43-4160, 45-14039, etc. Because of the advantages of relatively simple design of devices, easy maintenance and lack of noise, the recording devices employing such thermosensitive recording materials are being used in a wide field including recording instruments for measurements, facsimiles, printers, terminal devices for computers, labels, and automatic vending machines for railroad tickets and the like. Particularly in the field of facsimiles, the demand for thermal sensitive mode has been greatly increasing and the performance of facsimiles has becoming high speed due to reduction in transmission costs. Facsimiles have reduced the cost and minimized the energy consumption. In response to such high speed and low energy performance required for facsimiles, high sensitivity has been demanded for thermosensitive recording materials. On the other hand, dot density of a thermal head was generally 8 lines/mm but recently a density as high as 16 lines/mm has been used. In addition, a dot area has become small and, demands for printing small-sized characters in high image quality or printing characters with density gradation by Dither method have been increasing. Thus, good printability, namely, to obtain images faithfully reproduced from dots on a head has been in greater demand than ever.
Attempting to satisfy these requirements, adhesion between a recording sheet and a thermal head was improved by supercalendering to a strong degree but such a treatment resulted in defects of decreasing whiteness, i.e., so called background stain, and the like.
It is proposed in Japanese Patent Application KOKAI (Laid-Open) No. 56-27394 to provide an undercoat layer between a thermosensitive layer and the base paper. By the provision of an undercoat layer, high density images can be obtained in a small energy without any violent supercalendering and higher density can be achieved than before. It is believed that the provision of this undercoat layer would be effective for rendering the surface of a thermosensitive layer smooth after it is coated, by filling up unevenness of a support to provide a smooth surface.
As described above, by the provision of an undercoat layer, a higher density recording has been obtained than was known before. However, demands for much higher sensitivity and more improvement in the dot reproducibility in recent years cannot be dealt with simply by providing an undercoat layer merely aiming at smoothing the surface.